The invention relates to protein separation and purification methods. More specifically, the invention relates to the separation of alpha-1-antitrypsin (AAT, also known as alpha-1 proteinase inhibitor, API, and A1-PI) from complex protein mixtures such as blood plasma fractions, and to methods for further purification of the separated AAT so as to provide a composition suitable for pharmaceutical use.
Alpha-l-antitrypsin (AAT) is a glycopeptide inhibitor of proteases, and is found in human serum and other fluids. Protease inhibition by AAT is an essential component of the regulation of tissue proteolysis, and AAT deficiency is implicated in the pathology of several diseases. Individuals who inherit an alpha-1-antitrypsin deficiency, for example, have increased risk of suffering from severe early-onset emphysema, the result of unregulated destruction of lung tissue by human leukocyte elastase. The administration of exogenous human AAT has been shown to inhibit elastase and is associated with improved survival and reduction in the rate of decline of lung function in AAT-deficient patients (Crystal et al., Am. J Respir. Crit. Care Med. 158:49-59 (1998); see R. Mahadeva and D. Lomas, Thorax 53:501-505 (1998) for a review.)
Because of its therapeutic utility, commercial AAT production has been the subject of considerable research. Much progress has been made in the production of recombinant AAT in E. coli (R. Bischoff et al., Biochemistry 30:3464-3472 (1991)), yeast (K. Kwon et al., J. Biotechnology 42:191-195 (1995); Bollen et al., U.S. Pat. No. 4,629,567), and plants (J. Huang et al., Biotechnol. Prog. 17:126-33 (2001)), and by secretion in the milk of transgenic mammals (G. Wright et al., Biotechnology, 9:830-834 (1991); A. L. Archibald, Proc. Natl. Acad. Sci. USA, 87:5178-5182 (1990)). However, isolation of AAT from human plasma is presently the most efficient practical method of obtaining AAT in quantity, and human plasma is the only FDA-approved source.
A number of processes for isolating and purifying AAT from human plasma fractions have been described, involving combinations of precipitation, adsorption, extraction, and chromatographic steps. In order to minimize the risk of pathogen transfer, pooled human plasma intended for production of human AAT for therapeutic use is screened for the hepatitis B surface antigen, and for antibodies to the human immunodeficiency virus. As an additional precaution against transmission of infectious agents, the purified product is ordinarily pasteurized by heating to 60° C. for 10 hours (Mitra et al., Am. J. Med. 84(sup. 6A):87-90 (1988)) and sterile filtered.
Most published processes for AAT isolation begin with one or more fractions of human plasma known as the Cohn fraction IV precipitates, e.g. Cohn fraction IV1 or fraction IV1-4, which are obtained from plasma as a paste after a series of ethanol precipitations and pH adjustments (E. J. Cohn et al., J. Amer. Chem. Soc., 68:459-475 (1946)).
U.S. Pat. No. 3,301,842 describes a method for isolation of AAT from Cohn fraction IV1 wherein an acridine or quinoline derivative is added to the paste in a buffer at pH 6, the precipitate is discarded, and the pH adjusted to 7.0. Additional acridine or quinoline is added, and the precipitate is collected. This precipitate is dissolved in a pH 5.0 buffer, sodium chloride is added, and the resulting precipitate discarded. The solution, containing the AAT, is further processed by methanol precipitation. Alternatively, ammonium sulfate precipitations at pH 8 and at pH 5 are conducted with plasma, with the pH 5 supernatant being farther processed as above with quinoline or acridine additives.
Glaser et al., Preparative Biochemistry, 5:333-348 (1975), disclosed a method for isolating AAT from Cohn fraction IV1 paste. The paste is stirred in a phosphate buffer at pH 8.5 in order to reactivate the AAT, which is largely deactivated by the pH of 5.2 employed in the Cohn fractionation. After dialysis and centrifugation, the supernatant is subjected to two rounds of anion exchange chromatography at pH 6.0 to 7.6 and at pH 8.6, followed by further chromatographic processing at pH 7.6 and at pH 8.0, to produce AAT in about a 30% overall yield.
M. H. Coan et al., in U.S. Pat. Nos. 4,379,087 and 4,439,358 (see also M. H. Coan et al., Vox Sang., 48:333-342 (1985); M. H. Coan, Amer. J. Med., 84(sup 6A):32-36 (1988); and R. H. Hein et al., Eur. Respir. J., 3(sup 9):16s-20s (1990)), disclosed a procedure wherein Cohn fraction IV1 paste is dissolved in a pH 6.5 to 8.5 buffer, polyethylene glycol is added, and the pH is lowered to the range of 4.6 to 5.7 to precipitate unwanted proteins. After centrifugation, AAT is isolated from the supernatant by anion exchange chromatography. Further processing provides a 45% yield of AAT with a purity of about 60%. Methods employing polyethylene glycol as a precipitant are also described in U.S. Pat. Nos. 4,697,003, 4,656,254, and Japanese patent JP 08099999, described below; and also by Hao et al., Proc. Intl. Workshop on Technology for Protein Separation and Improvement of Blood Plasma Fractionation, Sep. 7-9, 1977, Reston, Va.
Dubin et al., Preparative Biochemistry. 20:63-70 (1990), disclosed a two step chromatographic purification, in which AAT was first eluted from Blue SEPHAROSE® and then purified by gel filtration chromatography.
Schultze and Heimburger, in U.S. Pat. No. 3,293,236, disclosed purification of AAT using cation exchange chromatography with a citrate buffer, in combination with ammonium sulfate fractionation of human plasma.
Lebing and Chen, in U.S. Pat. No. 5,610,285, disclosed a purification process which employs an initial anion exchange chromatography, followed by cation exchange chromatography at low pH and low ionic strength, to purify human AAT from plasma and plasma fractions. The cation chromatography takes advantage of the fact that active AAT does not bind to the ion exchange column under these conditions while contaminating proteins, including denatured AAT and albumin, are retained.
Jordan et al., in U.S. Pat. No. 4,749,783, described the isolation of AAT from human plasma using affinity chromatography with monoclonal antibodies. See also Podiarene et al., Vopr. Med. Khim. 35:96-99 (1989).
Shearer et al., in European patent application EP 0 224 811 and in the corresponding U.S. Pat. No. 4,656,254, disclosed an improved method for extracting AAT from Cohn fraction IV paste, in which the improvement consisted of treating the paste with a larger volume of buffer, at a higher pH, than had been customary in the prior art. The combination of higher volume and higher pH increased the amount of AAT extracted from the paste. Unwanted proteins were precipitated by addition of polyethylene glycol, followed by centrifugation. An alternative procedure is disclosed, which is essentially the procedure of Coan et al., wherein after addition of polyethylene glycol, the pH is adjusted to the range of 4.6 to 5.7, and the acidified mixture held for from one to sixty minutes to further precipitate unwanted proteins. The AAT is precipitated by addition of additional polyethylene glycol, and further purified by anion exchange chromatography.
Arrighi et al., in European application EP 0717049, disclosed a process wherein fraction IV1 paste is stirred in a pH 8.2 buffer at 40° C. for one hour, followed by precipitation of unwanted proteins with ammonium sulfate. The AAT is isolated from the supernatant by hydrophobic interaction chromatography at pH 7.
Kress et al., in Preparative Biochemistry 3:541-552 (1973), dialyzed the precipitate from an 80% ammonium sulfate treatment of human plasma, then chromatographed it on DEAE-cellulose. The product was dialyzed again and gel filtered on SEPHADEX™ G-100. AAT-containing fractions were then chromatographed on DE-52 cellulose to give AAT.
Japanese patent 59-128335 discloses the precipitation of unwanted proteins from a plasma fraction by addition of polyethylene glycol at a pH between 5 and 7, followed by anion exchange chromatography.
Bollen et al., in U.S. Pat. No. 4,629,567, disclose the isolation of AAT from a culture of yeast carrying recombinant plasmids expressing AAT. The process begins with polyethylene glycol precipitation at pH 6.5 to remove contaminating proteins, followed by anion exchange chromatography at pH 6.5 and subsequent chromatographic steps.
Dove and Mitra, in U.S. Pat. No. 4,684,723, disclose a variant of the method of Coan et al. (U.S. Pat. Nos. 4,379,087 and 4,439,358) in which AAT is purified by a process comprising the steps of (a) holding a solution containing AAT at a pH of 6.5 to 8.5 for up to 24 hours, (b) adding polyethylene glycol and an inorganic salt, so as to obtain a two-phase mixture, and (c) isolating the aqueous salt phase, which contains purified AAT.
Taniguchi et al., in PCT application WO 95/35306, disclose a similar process, involving precipitation with polyethylene glycol in the presence of zinc chloride, followed by anion-exchange chromatography and chromatography on a metal chelate resin.
Van Wietnendaele et al., in U.S. Pat. No. 4,857,317, also disclose a process for isolating AAT from the crude extract of an engineered yeast culture, which comprises addition of polyethylene glycol at pH 6.1, centrifugation to remove precipitated proteins, addition of calcium chloride, storage for 24 hours at pH 7.0, and centrifugation to further remove contaminants. AAT is then isolated from the supernatant by subsequent chromatographic steps.
Coan, in U.S. Pat. No. 4,697,003, discloses a method for isolating AAT from various Cohn plasma fractions which comprises the removal of ethanol and salts from an AAT-containing fraction, followed by anion-exchange chromatography on DEAE cellulose or a similar material under conditions such that the AAT is retained on the column while undesired proteins are eluted. Coan also describes “pasteurization” at about 60° C. or more for about 10 hours, which is stated to be sufficient to render hepatitis viruses non-infective.
Coan discloses addition of carbohydrate as a stabilization agent, either alone or with sodium citrate, in order to stabilize the AAT at the pasteurization temperature. Suitable carbohydrates are said to be mono-, di-, and trisaccharides, and sugar alcohols such as sorbitol and mannitol. AAT is prone to both polymerization and to the adoption of inactive conformations upon heating; the presence of stabilizers reduces but does not eliminate thermal inactivation (D. Lomas et al., Eur. Resp. J. 10:672-675 (1997)). Size-exclusion HPLC analysis has shown that 10% of monomeric AAT is polymerized or aggregated when pasteurization is carried out according to the Coan process (M. H. Coan et al., Vox Sang., 48:333-342 (1985)).
T. Burnouf et al., Vox Sang., 52:291-297 (1987), disclosed substantially the same procedure for isolating AAT from Kistler-Nitschmann supernatant A. DEAE chromatography of Cohn Fractions II+III and size exclusion chromatography produced an AAT which was 80-90% pure (by SDS-PAGE) with a 36-fold increase in purity. Recovery was 65-70%.
Thierry, in European patent application EP 0282363, also discloses a method of obtaining AAT from a Kistler-Nitschmann plasma fraction. Briefly, plasma is precipitated with 10% ethanol at pH 7.4, and the supernatant precipitated again with 19% ethanol at pH 5.85. The supernatant from the second precipitation is applied to a DEAE anion-exchange column, and eluted at pH 5.2 to provide AAT of about 90% purity.
Strancar et al., in PCT patent application WO 95/24428, disclose a very similar method, employing a particular class of functionalized anion-exchange media. Desalted Cohn fraction IV1 is applied to the column, and contaminating proteins are eluted with low salt buffer at a pH “close to the pKa of acetic acid.” (The pKa of acetic acid is 4.74.) AAT is then eluted with 50 to 300 mM NaCl at pH 7.4 to 9.2.
Japanese patent JP 08099999 discloses a method of obtaining AAT from Cohn fraction IV or IV1, which involves reduction of salt concentration to below about 0.02 M, adjusting the pH to 4.5 to 5.5, and contacting the solution with a cation exchanger to adsorb contaminating proteins.
M. E. Svoboda and J. J. van Wyk, in Meth. Enzymology, 109:798-816 (1985), disclose acid extraction of Cohn fraction IV paste with phosphoric, formic, and acetic acids.
Glaser et al., in Anal. Biochem., 124:364-371 (1982) and also in European Patent Application EP 0 067 293, disclose several variations on a method for isolating AAT from Cohn fraction IV1 precipitate which comprises the steps of (a) dissolving the paste in a pH 8.5 buffer, (b) filtering, (c) adding a dithiol such as DTT, and (d) precipitation of denatured proteins with ammonium sulfate. Glaser states that the destabilized (DTT-reduced) proteins may be precipitated by “suitable techniques such as salting, heating, change in pH, addition of solvents and the like.”
Glaser et al. describe one variation in which treatment with DTT is carried out in the presence of 2.5% AEROSIL® fumed silica, prior to precipitation with 50% saturated ammonium sulfate. Recovery of AAT was as good as it was in the absence of the silica, and the purification factor was improved by about 70%. In both references, the authors relegate the silica to a secondary role, that of an additive that improves the results of the ammonium sulfate precipitation. The effectiveness of silica alone, without ammonium sulfate precipitation, is not recognized or described. If the concentration of the protein solution appreciably exceeds about 50 mg protein/ml, AAT is reportedly lost by occlusion in the precipitate.
Ralston and Drohan, in U.S. Pat. No. 6,093,804, disclose a method involving the removal of lipoproteins from an initial protein suspension via a “lipid removal agent,” followed by removal of “inactive AAT” via elution from an anion-exchange medium with a citrate buffer. The lipid removal agent is stated to be MICRO CEL® E, a synthetic hydrous calcium silicate. In the presence of a non-citrate buffer, the anion-exchange medium binds active AAT while allowing “inactive AAT” to pass through. A citrate buffer is specified for subsequent elution of the AAT from the anion exchange medium, and also for later elution from a cation-exchange medium. Ralston and Drohan do not describe the use of a disulfide-reducing agent. The process is stated to provide AAT with a product purity of >90%; and manufacturing scale yields of >70%.
W. Stephan, in Vox Sanguinis 20:442-457 (1971), describes the use of fumed silica to adsorb lipoproteins from human blood serum solutions. The effect of silica adsorption on the concentrations of several plasma proteins, including AAT, was evaluated, and there was no significant loss of AAT.
Mattes et al., in Vox Sanguinis 81:29-36 (2001), and in PCT application WO 98/56821 and published US patent application 2002/0082214, disclose a method for isolating AAT from Cohn fraction IV which involves ethanol precipitation, anion exchange chromatography, and adsorption chromatography on hydroxyapatite. The latter step is reported to remove inactive AAT, providing a product with very high specific activity.
While AAT is an effective treatment for emphysema due to alpha-1-antitrypsin deficiency, treatment is very costly (currently about $25,000 per year), due to the limited supply and a complex manufacturing process. There remains a need for more efficient and cost-effective methods for isolating human AAT from plasma and other complex protein mixtures containing AAT. In particular, ammonium sulfate precipitation followed by dialysis is a time-consuming process, that generates substantial amounts of waste water, and there is a need for scalable processes that do not require extensive dialysis while providing high yields of high activity, high purity AAT. Thermal pasteurization of AAT effectively reduces viral contamination, but it leads to the formation of inactive AAT aggregates and polymers. Thus, there is also a need for highly pure AAT with reduced viral contamination but without significant amounts of inactive AAT aggregates and polymers. The present invention addresses these needs.